Clarify need for Access Rule in mobile-settings.js

[asidiali]

Hey there!

Just thought I'd share, it's not really obvious that icons won't appear in mobile builds unless you specify an access rule in your mobile-settings.js....otherwise the resource doesn't get loaded.

Maybe just a note about it would be useful, including the access rules in your readme:

App.accessRule('http://code.ionicframework.com/ionicons/');
App.accessRule('https://code.ionicframework.com/ionicons/');

See: https://forums.meteor.com/t/ionicons-disappearing-on-mobile-app/12579